In German Patent document DE 41 31 365 and corresponding to commonly assigned application Ser. No. 07/948,157, filed 21 Sep. 1992, now abandoned, a process for travelling lateral deviation of a travelling web from a setpoint position or desired position is described which involves determining the actual position (instantaneous position). The actual or instantaneous position is then compared with the setpoint position and a setting signal is formed for the operation of the effector which influences the fabric web position in dependence upon the deviation of the actual position from the desired or setpoint position.
In that system, an image/signal converter, especially a video camera, is used to take a picture or acquire an image of at least one longitudinal strip through which a guide criterium of the web runs in the longitudinal or travelling direction of the web.
The travelling web, of course, moves past the image/signal converter and thus the image of the successive strips of the web is repetitively captured or acquired cyclically. For each image acquisition the image can be stored and from it, by signal evaluation or processing, the actual position of the guide criterium is determined and used as an actual value signal for controlling the position of the web in the position correction process.
The process described in the above described application and German patent document, utilizes a guide criterium which can be an edge formed by a jump in the web thickness.
It is also known to use for guiding a travelling web, a boundary edge itself or a continuous line which can be, for example, printed on the fabric web.
All of these conventional processes utilizing continuous guide criteria have in common that the signal image acquired by the image/signal converter can only use flank spacers and flank orientations to determine the actual position. That, of course, means that the guide criterium must generate in the signal image clear and sharply delineated flanks. Because of this requirement, there are only a few structures or patterns in or on the web which are suitable to serve as guide criteria.